1. Field of the Invention
This invention relates to a lens, and more particularly to a rear stop type lens whose comatic flare is corrected to a high degree without aggravating other aberrations.
2. Description of the Prior Art
In the lens wherein the diaphragm is positioned to the rear of the lens, the asymmetry is considerably greater than in a lens of the type which has a diaphragm inside the lens system. As a result, the correction of aberrations in this type lens becomes extremely difficult. Particularly, the correction of comatic aberration of the oblique ray passing through the lower half portion of the diaphragm becomes a difficult problem. The reason is as follows. In the ordinary lens, the range of luminous flux used can be freely selected for maximum advantage in correction and the luminous flux itself can be controlled by stopping down the diaphragm. In contrast, in the rear stop type lens, the oblique ray is utilized down to a much lower portion than in the ordinary lens for the purpose of precluding the phenomenon of shading due to the stopping down of the diaphragm (particularly in the type of camera wherein, by reason of the camera structure, the lens only is set forward with the diaphragm left behind) and, as a result, the correction of comatic aberration becomes difficult. In addition, the width of the luminous flux in the lower half portion of the diaphragm tends to decrease when the stop is opened to its full extent. When comatic flare remains in this portion, it cannot be eliminated in the commonly used range of from F/5.6 to F/8. Consequently, the image contrast is seriously impaired. Particularly since program type shutters are often used in the medium-priced cameras in which the lenses are mainly used, the photographer is not allowed to control the aperture as he wishes. Since the diaphragm in such a camera is kept open very widely even under ordinary brightness, comatic flare is aggravated to the point of causing a serious problem.
Heretofore, among the rear stop type lenses of this kind, those of Tessar configuration have had the widest use. Lenses of Tessar configuration, however, are disadvantageous because the second and third surfaces generate strong divergence and because they tend to cause the aforementioned comatic flare of the luminous flux in the lower half portion of the diaphragm since the third surface is strongly concave in the direction of the object. This comatic flare cannot easily be eliminated by merely stopping down the diaphragm. As a result, the image contrast is seriously impaired.